As known in the art, a primary consideration in any digital communications system is the channel bandwidth and channel separation required to transmit information. Therefore, digital systems are typically designed to utilize channel bandwidth as efficiently as possible. For example, in systems utilizing frequency division multiplexing, maximum spectral efficiency is obtained by spacing frequency channels very close to one another in an available spectrum.
Minimum carrier spacing is limited in practice, however, by adjacent channel interference. Adjacent channel interference is defined as the interference resulting when carrier frequencies are spaced close enough to one another that information signals received from number of carriers overlap in the frequency spectrum. In practice, the minimum allowable carrier spacing is a function of the bandwidths of the information signals, the practical limitations associated with receiver filtering, and the signal modulation and coding schemes used. Number of design improvement providing increased suppression of adjacent channel interference have been suggested to increase system capacity, relax coding and modulation design requirements, or improve signal quality.
In conventional systems, adjacent channel interference is suppressed in a number of ways. For example, in certain cellular radio systems, adjacent channel interference is avoided through channel allocation schemes in which channels immediately adjacent to one another in frequency are assigned to different spacial cells. Consequently, physical separation reduces mutual interference between adjacent channels. In other communications systems (e.g., satellite and land mobile radio systems), however, suppression of adjacent channel interference by physical separation of adjacent channels may not be possible.
By an alternative conventional approach, during demodulation of a given carrier signal, a bandpass filter centered at an adjacent carrier is used to extract an adjacent channel signal (ACS) at the adjacent carrier. The extracted signal is then used to estimate the adjacent channel signal envelope and carrier and to coherently detect the adjacent channel signal. The detected adjacent channel signal is then waveform shaped, and the estimated adjacent channel carrier and envelope are impressed on the resulting signal. Ideally, the described process provides a reconstructed adjacent channel signal at its carrier frequency. The reconstructed signal can then be passed through a bandpass filter centered at the carrier of interest and subtracted from the received signal to remove the adjacent channel interference.
Such an approach has several limitations, however. For example, analog signal processing using filters and mixers adds undesirable cost and size to a radio receiver, and since the analog components vary with the manufacturing process, such receivers provide a relatively unpredictable range of performance. Additionally, subtracting a signal at radio frequency requires highly accurate carrier reconstruction and time alignment, as an error as small as half a cycle at radio frequency can cause the adjacent channel signal to double rather than diminish. Furthermore, such use of the adjacent channel carrier (phase and frequency) and envelope (amplitude) implicitly assumes that the radio channels are not dispersive. However, in many practical wireless systems (e.g., D-AMPS and GSM), the symbol rate is sufficiently high that the radio transmission medium must be modeled to include time dispersion which gives rise to signal echoes. Thus, the proposed technique is not always practical for use in many present day applications.
According to another conventional approach, demodulation parameters such as linear or decision feedback equalization filter coefficients are adapted to minimize noise and adjacent channel interference together. Alternatively, spectrally efficient continuous phase modulation (CPM) techniques can be used to reduce the effects of adjacent channel interference.
U.S. Pat. No. 6,108,517 discloses methods and apparatus for receiving adjacent channel signals wherein adjacent channel interference effects are minimized through joint demodulation of the adjacent channel signals. A channel associated with each signal and each corresponding frequency band is estimated and used to form joint branch metrics for joint sequence estimation. Thus, a baseband processor receives baseband samples corresponding to a certain carrier frequency, and then jointly demodulates at least two information streams corresponding to different carrier frequencies in dependence upon the received baseband samples.
U.S. Pat. No. 5,710,797 describes a single transducer digital communication receiver which is capable of extracting the data bits of at least one desired signal in the presence of interfering signals of similar type, so as to provide a system in which overlapping transmissions are tolerated and allowed. This publication describes a specific frequency plan, which includes an appropriate digital demodulator that extracts the data bits of the desired signal(s) in the presence of closely spaced signals. The disclosure enables reduced channel spacing in digital communication systems and thereby increases the system capacity (i.e. the number of users per bandwidth unit) without incurring any significant loss in system performance (e.g. power margins, BER, and channel availability). It also allows a reduced power margin that is required to maintain a pre-specified performance level without sacrificing system capacity.
The disclosure of the references mentioned herein throughout the present specification are hereby incorporated by reference.
As noted above, however, minimizing or avoiding adjacent channel interference using the above described systems provides relatively minor improvement with respect to spectral efficiency, and current suppression mechanisms are inadequate for broad applications. Thus, there is a need for improved methods and apparatus for significantly reducing the impact of adjacent channel interference.